1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof, and, more particularly relates to a semiconductor device including a trench-gate transistor and a manufacturing method thereof. The present invention also relates to a data processing system configured by such a semiconductor device.
2. Description of Related Art
In recent years, downscaling of high density and lower power DRAM (Dynamic Random Access Memory) requires a very short gate length of a cell transistor. The short gate length of the transistor leads to short channel effect degradation which results in high leakage current of a DRAM chip. Thus, the DRAM is confronted with the serious problem of this short channel effect.
To overcome this problem, attention is paid to a so-called trench-gate transistor (also called a “recess-channel transistor”) so configured that a gate electrode is buried in a groove (trench) formed on a semiconductor substrate. By using the trench-gate transistor, an effective channel length can be sufficiently secured and a fine DRAM having a minimum machining dimension of 90 nm or less can also be realized. Further, there has recently been proposed a method in which a thin SOI (Silicon On Insulator) structure within a recess is formed, and the silicon layer is used as a channel region (Japanese Patent Application Laid-open No. H8-274277).
When the cell transistors shrink aggressively, there arises a need of narrowing the width of the gate trench. Thus, as shown in FIG. 15, a curvature radius at a corner on a bottom of a gate trench 16 becomes small, and thus a sub-threshold coefficient increases. This is probably due to the fact that a ratio of a channel-depletion-layer capacitance to a gate-oxide capacitance becomes greater at a location where the curvature radius is small. Compared to the larger curvature radius of the corner of the trench, the stand-by current of the smaller one increases. As a result, there is a problem that when the width of the gate trench is narrower, the refresh characteristic deteriorates.